The present invention relates to the ribbon-type loudspeaker that has the characteristics of large effects of heat radiation such that electrical input power is increased.
In the prior art, the diaphragm of the ribbon-type loudspeaker must meet two requirements: light-weight and large conductivity. To meet these two requirements, there has conventionally been used ribbon-type diaphragm made of conductive metal foil alone such as aluminium foil or ribbon-type diaphragm made of high-polymer film with its one side or both sides formed with conductive portion.
On the other hand, the effective cooling of the diaphragm has been required, because allowable electrical input power for the loudspeaker must be increased. Without effective cooling a ribbon-type diaphragm is fused due to heat generated therein (Joule's heat). Or, the input power is apt to subject the ribbon-type diaphragm to thermal deformation before fusion takes place.
In the prior art ribbon-type diaphragm, heat propagation takes place through three paths as shown in FIG. 1.
The path A conducts heat directly from the diaphragm to the air therearound. The path B conducts heat to the plates 4 of the magnetic circuit 2 and the metalic frame (not shown) via the viscoelastic seal elements 5 holding both ends of said diaphragm 1'. The path C is space around the diaphragm through which heat radiates directly from the diaphragm. In this way, cooling of the diaphragm takes place
Among these three paths, both path B that conducts heat from the diaphragm 1' to the plates 4 of the magnetic circuit 2 via seal elements 5 and path A that conducts heat directly to the air from the oscillating element 1' are main passages of heat propagation at a low temperature of the diaphragm 1'.
For example, as shown in FIG. 1, the path B that propagates heat due to heat conduction to the plates 4 of the magnetic circuit 2 via seal elements 5 contributes to greater heat propagation than the path C.
On the other hand, at a high temperature of the diaphragm 1', the other path C that propagates heat by radiation works more efficiently than others.
Now, heat propagation Q through the path C is indicated in the following general equation; EQU Q=A.epsilon..delta.T.sup.4
where A is an effective area emanating (or absorbing) heat, .delta. is Stephan-Boltzmann's constant, and T is the absolute temperature of A [.degree.K].
Although large emmissivity .epsilon. is needed in order to obtain large Q, in the very case of the metal used generally to the oscillating element of the ribbon-type loudspeaker such as aluminium or the like, the generally-obtained value of .epsilon. is as low as 0.04.